Apparatus for counting particles



May 7, 1957 Filed June 25, 1952 H. A. DELL ET AL APPARATUS FOR COUNTINGPARTICLES 5 Sheets-Sheet 1 q mg J SAMPLE 9 f P/c/r-l/P $CA/VNER 4AMPLIFIER sou/m 5 g 1 sw/rcH b7 5 u/wr F HT 4- ,i! m AHPLIf/ER I s arWOBBLE spor-wosazs 4/ v \15 FROM P/C/rup 45/ :6

INVENTORS Emlyn Jones AGENT May 7, 17957 H. A. DELL ETAL APPARATUS FORCOUNTING PARTICLES Filed June 25, 1952 SWITCH CONTROL V BUFFER FILTERSWITCH CONTROL sumo 3 5 Sheets-Sheet 2 FORWARD COUNTER BAG/ WARDINVENTORS Emlyn Jones Hugh Alexa? Dell BY W 17 AGENT May 7, 1957 H. A.DELL ETAL APPARATUS FOR coum'mc PARTICLES 5 Sheets-Sheet 3 Filed June25, 1952 team SWITCH ca/mzouza. cowl-Pa GENERATO i/ f INVENTORS H EmlynJoges I g exo er e BY AGENT May 7, 1957 H. A. DELL ET AL 2,791,377

APPARATUS FOR COUNTING PARTICLES Filed June 25, 1952 5 Sheets-She'et 4 4J D/FF 9 v v J Q' PULSE come/b.5022 coy/77E? SCANNER $5411 7 i. .sw/reh'&

Il INVENTORS Emlyn Jones A Hugh Alexond Dell BY;%(WW

AGENT May 7, 1957 H, A, DELL ETAL 2,791,377

APPARATUS FOR COUNTING PARTICLES Filed June 25,- 1952 5 Sheets-Sheet 5BLEL'ELL 27 1 J3 JB\ 5 58 J0 Q26:

1 MEL'I'MIYICAL 4 gum 5 I JYbTEM 6 :9 5

R ffilli. ECELL ,1 1, ear.

lNVE'NTORS Emlyn Jones Hugh Alexander Dell AGENT United States PatentAPPARATUS FOR COUNTING PARTICLES 5 Hugh Alexander Dell and Emlyn Jones,Harley, England, assignors, by mesne assignments, to North AmericanPhilips Company, Inc, New York, N. l., a corporation of Delaware Theinvention relates to apparatus for counting particles and isparticularly but not exclusively concerned with the assessment of thedust content of an air sample.

Assessment of the contamination of air or other gas by dust has hithertobeen effected by preparinga sample taken under controlled conditions,such sample being for example a transparent plate on which the dustparticles have settled and been fixed, or an enlarged photograph of sucha plate, visually examining the sample under a microscope and countingthe particles in a representative area. This is a long and laborioustask and the results obtained from the same sample by difierentobservers may vary widely particularly when the particles vary greatlyin size.

For certain purposes it is only necessary to have. a total count of thenumber of particles in a given sample without size discrimination, andthe object of the invention is to replace visual counting by automaticelectrical counting apparatus giving such a total count.

If the particles in a given sample vary in size then there remains thedifficulty, if some form of scanning of the sample is used, of ensuringthat a large particle scanned more than once is only seen as a singleparticle so that a spurious count is avoided.

A further object or the invention is to ditficulty. v

With these and other objects in view and according to the invention,particle counting apparatus comprises means such as a cathode ray tubefor scanning a sample of the remove this particles to be counted,pick-up means such as a photoy electric ccll co-operating with thescanning means for producing an electrical signal which is a measure ofthe presence and distribution of the particles, means, for avoidingmultiple mounting of a large particle scanned more than once andcounting means responsive to the derived signal or signals for giving anindication of the total number of particles scanned.

When the sample is in the form of a transparent plate bearing theparticles or a photographic representation of the same to the same or adifferent scale the scanning means may comprise a cathode ray tube, thebeam of which is caused, by suitable saw-tooth time bases, to trace outa raster of rectangular form and of such size as to illuminate the plateor such part of it as it is required to examine. By suitable opticalmeans the, light from the cathode ray tube face passing through thesample-is caused to fall on a pick-up device such as a photo-cellPatented May 7, 1957 is actuated when one only of said beams encountersa particle.

With such an arrangement three modes of utilising the signals providedby the pick-up means may be distinguished. In the first mode the signalderived from one beam is employed, for example after inversion, tocancel the signal derived from the other beam when both beamssubstantially simultaneously encounter a particle overlapping :two ormore lines of scan. Counting of such a particle or a small particleoccupying only one scanning line may be elfected either-- (a) Byutilising the signal derived from one of the beams when it alone scansthe particle so that counting is effected either when the particle isfirst encountered by one of the beams or when it is last encountered bythe other beam, or -(b) By utilising the signals from both beams (whenthey are not present substantially simultaneously) so that a particle iscounted when it is first encountered by one of .the beams and when it islast encountered by the other beams, the total count thus obtained beingdivided by two.

In the second mode the electrical signals derived from the two beams maybe of similar electrical polarity (for example positive going pulses)and are mixed so that when each beam only encounters a particle, a firsttype (or waveform) of signal is obtained whereas when both beamssubstantially simultaneously encounter a particle a different type (orwaveform) of signal results. Such a different type of signal may beutilised to prevent actuation of the counter. In this case each particlewill be counted twice as in the second of the regimes of the I firstmode so that the total count must be divided by two.

In the first mode the signal provided by one beam may be regarded as acounting signal and the signal provided by the other beam as acontrolling signal whereby the transmission of the counting signal tothe counter (when a particle is encountered) is prevented when the twosignals are present substantially simultaneously but is permitted whenthe counting signal only is present. In this case counting may takeplace either when the particle is first encountered or when it is lastencountered whichever is most desirable in a particular case.

Since large particles overlapping two or more scanning lines may presenta leading edge which is not normal to the direction of line scan, it ispossible to arrange that one of said beams, for example the leading beamin the direction of frame scan, which may be termed the guard beam,should encounter the leading edge of the particle in advance (in thedirection of line scan) of the other beam (the trailing beam in thedirection of frame scan) which may be called the scanning or countingbeam so that the output from the pick-up due to the guard beam shall beoperative to prevent actuation of the counter slightly before thescanning beam encounters the leading edge of the particle.

According to a further feature of the invention there i fore the samplescanning means provides effectively two so that the output of the pickupdevice in time is an electrical representation of the presence orabsence of particles in the scanning lines.

Since the particles may vary in size it is possible that a largeparticle may overlap two or more lines ofscan and according to a furtherfeature of the invention the sample scanning means provides effectivelytwo scanning beams scanning adjacent lines, the PiQk-llp means andassociated amplifier or amplifiers and counting means being so arrangedthat when both beams encounter a spot in an appropriate direction and tothe desired extent and at a suitable repetition frequency so thatefiectively two scanning beams are .provided. In another form two flyingspot light beams having distinguishably diiferent optical properties andscanning adjacent lines may be used.

particle the counting means is rendered inoperative-and In accordancewith a further feature of the invention therefore, the sample scanningmeans comprises a flying only of said scanning beams. The flying spotlight beam 1 may be provided in the known manner by a flying spotscanner of the cathode ray tube type or by a light source and mirrordrum or drums or equivalent opticalelcments and the beam may be dividedby any suitable optical element or elements, the beam differentiatingmeans, comprising colour filters or polarising means, the photo-electricdevices being correspondingly colour or polarisation sensitive. I Otherfeatures of the invention will be apparent from Alternatively the samplemay be for example in the form of a photographic print from which lightis reflected into the pick-up device and may be either a positive ornegative image, that is to say the particles may appear as black markson a white ground or as white marks on a black ground.

Apparatus for scanning such a sample and operating according to thefirst mode above mentioned is shown in one embodiment in Figure 2 inblock schematic form. The scanning unit 3 comprises a flying spotscanner'of the cathode ray tube type, having the usual saw-tooth lineand frame time bases to cause the electron beam to produce a rectangularraster on the tube face. Associated with the cathode ray tube is apick-up device 4 such as a photo-cell, the particle sample beinginterposed between the cathode ray tube face and the device 4 so that asthe 7 sample is scanned the photo-cell provides an electrical thefollowing description of several embodiments which are given by way ofexample only and with reference to the accompanying drawings in which:

Figure 1 is a representation of a portion of a particle sample;

Figure 2 is a block schematic diagram of particle counter;

Figure 3 is a circuit diagram of the switch unit of Figure 2;

of a first form Figure 4 is a block schematic diagram of a second form Iv of particle counter; a 1

Figure 5 is a block schematic of a third form of particle counter;

Figure 6 is a circuit diagram of the pulse selector switch and switchcontrol and count pulse generator units of Figure 5;

Figure 7 shows typical waveforms of signals applied to the circuit ofFigure 6;

Figure 8 is a block schematic diagram of a fourth form of particlecounter;

Figure'9 is a circuit diagram of the differentiator and pulse selector,and coincidence switch units of Figure 8;

Figure 10 shows typical waveforms of signals applied to the circuit ofFigure 9;

Figure 11 is a diagrammatic representation of oneform of opticalscanning system;

Figure 12 is a diagrammatic representation of a second form of opticalscanning system;

Figure 13'is a diagrammatic representation of a scansignal which is arepresentation of the presence or absence of particles in the scanninglines. These scanning lines are indicated in Figure 1 by the horizontallines a, b, c, d. With conventional single spot scanning and in theabsence of size discriminating means the large particle will be scannedthree times as the scanning beam traverses lines a, b and 0, thus givingrise to two spurious signals.

In order to avoid this and to provide such size discrimination, amodification of the known spot wobble technique used in television maybe employed as indicated in Figure 2. A unit 5 providing spot wobblingpotentials is connected to the scanning unit 3 and is so arranged thatthe cathode ray beam is deflected in the direction of frame scan by adistance equal to one line width. In addition and for a purpose whichwill be hereinafter described the cathode ray beam is also deflected inthe direction of line scan by a predetermined amount suflicient toensure that the beam in its deflected position first scans the side of aparticle sloping in the direction of line scan as shown at 6 in Figurel. The repetition frequency at which the cathode ray beam is deflectedis preferably many times the line frequency and in a practical examplewhen the line frequency is 1000 c./s. the spot wobble repetitionfrequency may be for example 1 mc./s. The alternating potential givingrise to this deflection is preferably of square waveform.

For the purpose of the following description the cathode ray beam in itsnormal undeflected position will be called the scanning beam and in itsdeflected position the guard beam and the scanning will be assumed to befrom left to right for line scan and top to bottom for frame ning systemutilising a modified scanning electron microscope.

In the following description of several practical embodiments of theinvention and with reference to the operation of certain thermionicvalves the phrase open on the control (or suppressor) grid means thatthe potential of the control (or suppressor) grid is such as to permitpassage of the electron stream. By the phrase cutoff on the control (orsuppressor) grid is meant that the How of electron is substantiallyreduced or entirely prevented.

scan with reference to Figure 1.

Referring again to Figure 2 the spot wobble unit 5 is connected to twoinput terminals of an electronic switch unit 7. The pick-up device isalso connected to another input terminal of the switch unit and twooutput terminals of the unit are connected respectively to two inputs ofan amplifier 8. These two inputs represent respectively'the signals dueto the scanning beam and guard beam and the amplifier is so arrangedthat when both signals are present one cancels the other thuseffectively quenching the amplifier. The amplifier output is connectedto a counter 9 of any suitableform.

A suitable circuit for the switch unit 7 is shown in a I, Figure 3 andcomprises two thermionic valves 10, 11,

It is also to be understood that the size of the scanning I spot at thesample under investigation is preferably not larger than the smallestparticle it is desired to count and may be smaller, and the distancebetween the centres of the scanning lines comprising the raster isnotless than the diameter of the scanning spot.

Referring now to the drawing, Figure 1 shows for the purpose ofsimplifying the following description a portion of a sample platecontaining only a small particle 1 and a large particle 2. The actualsample may be a transmeans.

parent plate having the particles adhering to it or it may be aphotographic reproduction in the form of a lantern slide of an actualsample to the same or 'a different scale.

each having two control electrodes. These valves may be of the pentodetype. The output of the photocell 4 is'connected to'the control grid 12of valve 10 and to the control grid 13 of valve 11 and the spot wobblingsquare wave potential to the suppressor grid 14 of valve 10 The samesquare wave potential inverted is supplied to the suppressor grid 15 ofthe valve 11. The output of valve 10 is supplied to the input of theamplifier and the output of the valve 11 to the amplifier quenching Inoperation and assuming the scanning beam to be seaming line a in Figure1 the output of the photocell will, for example increase as the scanningspot traverses the particle but this increase will be discontinuous dueto the repetitive deflection of the spot to the guard position. Valve 10will, therefore, pass a series of pulses since grids 12 and 14 of thevalve will go positive together but valve 11 will deliver no outputsince there is no output from the photocell in the guar position,although theinverted wave applied to the grid 15 of this valve tends toopen the valve at the appropriate instant. The output from valve 10 ispreferably integrated and supplied to the input of the amplifier as asingle pulse, which after amplification and squaring is supplied to thecounter.

As the scanning and guard beams approach the second, large particle, thephotocell will deliver an interrupted output due to the guard beam sincethis encounters the particle before the scanning beam and this photocelloutput will only appear in the output circuit of valve 11, valve 10remaining cut-01f. When the scanning beam encounters the particle therewill also be an output from valve 10 but by this time the output of thevalve ill will have reached a valve such that the output from valve 10is quenched so that the counter is not energised. Therefore, on itsfirst scan of the large particle (along line a Figure 1) the counter isinoperative and this applies also when line b is scanned since the guardbeam encounters the particle in line 0. On the next line, line d,however, the guard beam does not encounter the particle and the outputfrom the photocell as the scanning beam scans line 1: causes operationof the counter in the same manner as with the small particle.

While the large particle is being scanned by both beams (for examplelines a and b) the output of the photocell due to the guard beam mayterminate before the output of the cell due to the scanning beam becauseof the shape or disposition of the particle. This is undesirable sinceit may give rise to a spurious count. It can be avoided by the provisionof time delay means for example, in such a way that the amplifierremains quenched for a predetermined time after the termination of thecontrol potential due to the guard beam.

In an alternative arrangement not illustrated, the ou puts from valves19 and 11 may be difierentiated or otherwise converted in the knownmanner so that the output due to the scanning beam when it encountersthe particle appears as a single pulse of short duration correspondingto the leading edge ofthe particle which pulse is supplied to the inputof the amplifier as before. The single pulse due to the guard beam maybe used, for example, to trigger a flip-flop circuit which returns toits stable state after a predetermined interval. The operation of theflipflop circuit may be used to quench the amplifier and the time delaymay be such as to ensure that the scanning beam pulse has had time toappear. In this case the time delay may be much shorter than in thepreviously described arrangement.

in the above described arrangements the guard beam scans the linepreceding that scanned by the scanning beam (in the direction of framescan) and in the case of a large particle the count only takes placeafter the particle has been fully scanned. The inverse arrangement mayalsobe employed in which the guard beam scans the line succeeding theline scanned by the scanning beam (in the direction of frame scan). Inthis arrangement the particle is counted when it is first encountered bythe scanning beam, subsequent encounters, in the case of a largeparticle, producing no actuation of the counter.

A second embodiment of the invention operating according to the secondregime of the first mode is shown in block schematic form in Figure 4.In this arrangement the signal due to the main beam is taken to switchSM and the signal due to the guard beam is taken to a similar switch SGwhich switches are normally in the position shown in the drawing. Eachswitch is controlled by asignal fed into its associated switch controlunit (i. e.

SM at H and S6 at I). Outputs from these two switches are fed to acounter C capable of addition and substraction. The arrangement will bemore clearly understood from a consideration of the way in which itoperates. Considering the scanning of a large particle and assuming inthis case that the scanning beam is in advance of the guard beam in thedirection of frame scan, then, when the main beam encounters theparticle, a signal is routed to the counter via the switch SM and thecounter counts +1. Similarly whilst the signal lasts it is fed into theother switch control unit associated with switch SG at ;I and thusoperates switch S6 t9 its other position. On the first scan no signaldue to the guard beam is ofiered and when the signal due to main beamceases, switch SG reverts to the position shown in the drawing. On thenext scanning line and while assuming that themain beam encounters theparticle first, the arrangement works as on the first encounter so that+1 is added by the counter (making +2 in all) and switch SG is moved toits other position. The signal due to the guard beam now appears beforethe signal due to the main beam ceases and this guard beam signal is fedby the switch S G to the subtracting input of the counter which thusadds 1 (making +1 in all). This operation is repeated as long as bothbeams substantially simultaneously encounter the large particle untilthe scanning line is reached when only the guard beam gives a signal.This signal is routed to the switch control associated with switch SM atH and alsc to the counter which adds +1 (making +2 in all for the largeparticle).

In a similar way a small particle is counted twice so that the totalcount of the complete sample must be divided by two to arrive at thetotal number of particles.

it is to be noted that it does not matter whether the signal due to themain or guard beams appear first. Whichever it is adds +1 to the countand moves the others switch over so that when the other signal appearsit will add 1 and give the correct count.

A third embodiment of the invention operating according to the secondmode above mentioned, will'now be described with reference to Figure 5in which, to avoid elaboration, the scanning beam dividing and pick-upmeans are indicated as unit 16. They may be similar to those describedwith reference to Figure 2 or as described hereinafter.

From the pick-up means two electrical signals are produced, one due tothe scanning of a particle by the scanning beam and the other due to theguard beam as in the preceding embodiment.

In this embodiment however, each signal is of the same polarity and isdifferentiated in the known manner as indicated at 17 and 18 so that iffor example the output of the photocell due to the scanning beam as ittraverses a particle is a positive-going pulse of substantiallyrectangular form it is converted into a short positive going pulse (orspike) following by .a negative going pulse (or spike). (See Figure7a.)' The two differentiated signals are added together and theresulting mixed pulses are supplied to the input of a switch unit 11having outputs and to a pulse selector 20, the output of which goes toone input of a switch controller and count pulse generator unit 21. Twoother inputs to the unit 21 are connected to the switch unit 19 and afurther output from the unit 21 is taken to a counter 9 of any suitableform.

T he operation of this arrangement will be more readily followed from aconsideration of practical forms of the units 19, 2t) and 21 which willnow be described with reference to Figure 6. In this *figure the switchunit 1% comprises two thermionic valves V1, V2 which are of the pentodetype, the mixed signal above mentioned being supplied to the controlgrids of both valves. in the starting condition both valves are cut ofion their control grids, the valve V1 is also cut oif on its suppressorgrid and V2 is open on its suppressor grid due to starting condition. isso connected that the connection of these suppressor-grids to the switchcontrol andpulse generator unit'21r' A r This unit'comprises atriodeivalve' V3 and a pentode valve V4 which together form abi-stablemultivibrator, the anode of each valve being connected to the controlgrid of the other in the known mannerf Thefsuppressor grid of valve V1is connectedto the anode of valve V3 and the suppressor grid of valve;V2 is connected to the anode of valve V4. The anode of valve V1 isconnected to the control grid of valve V4 with av time delay network-R1Cr and the anode of valveV2 is, connected to the control grid ofvalve V3 withya'similar delay network 'R C2. In the startingconditionthe valve V3 is conduct- ..ing and valve V4 is cut off on itscontrol grid but is open on its suppressor grid. Ifnow a differentiatedsignalof the form shown in Figure 7(a) representing the scanning of asmall particle by one of the beams is applied to the control grids of,valves V1 and V2, the valve V2 conducts to its anode (-since it is openon its suppressor grid) whilst valve ,Vr remains cut off on itssuppressor grid. A negative -pulse is produced at the anode ofzvalve V2and this is icommunicated to the control grid of valve V3 which is nowcut oif, the rise of voltageon'its anode being communicated to thecontrol grid of'valve'V4 so causing this valve to conduct and themultivibrator has flipped to its other stable -stat,e.*Due to thisaction the valve V1 is now openedonits suppressor grid due to the riseof potential at the anode or V3 and the valve V2is now cutoff on itssuppressor gridas well as on its control grid. When the negative pulse(or spike) representing the trailing edge of the particle appears at thecontrol grids of valves V1 and V2; since both valves are cut ofi. 1

This pulse is however supplied through the pulse selector unit 20 whichcomprises diode V5 to the suppressor grid of valve V4 causing this valveto be cut off on its .suppressor grid. 7 e V A greater proportion of theemission current of this valve then goes to the screen grid producing anegative pulse which is communicated to the counter to register thepresence of the particle.

This action of the suppressor grid in substantially cutting otf theanode current of the valve V4 causes the rnultivibrator' V3, 'V4 to flopto its otherjstable state in which Va conducts and V4 iscut ofl? on itscontrol grid but open on its suppressor grid. This is accompanied by theresetting of the valves V1 and V2 to their original It is to be notedthat the diode V5 positive puls-es'are prevented from reaching thesuppressor grid of valve V4 and only negative going pulse-s have anyetfect on this electrode.

When a large particle is encountered by both scanning and guard beams awaveform of the general type shown .in Figure 7(b) is produced and theaction of the units .19, and 21 with such a signal will now bedescribed.

When the first positive going pulse representing the leading edge of theparticle, as seen by one of the beams,

appears on the control grids of valves V1 and V2, the

-circuits operate to the new stable state as already described above inwhich condition the'multivibrator V3,

-Vs-has flipped and valves V and V2 are both out otf.

'When the second positive pulse representing the leading edge of theparticle, as seen by the ot her beam, appears on the control grids ofvalves V1 and V2 the valve V1 conducts since it is open on itssuppressor grid, but

valve V2 remains cut otf on its suppress-or grid. The drop of potentialat the anode'of valve Vrcauses the valve Vito be cut olf on its controlgrid so that the multivibrator resets or flops to itsoriginalstablestate in I which Vs conducts. When the multivibrator is actuatedinQthis way no negative pulse'appears on the screen grid ,of. .valve V4.The resetting ofthe multivibrator causes Vi to be' cut otf on itssuppressor grid and V2 to be no action takes place passed to the controlgrid of valve Vs.

opened on its suppressor grid, both valves remaining cut ofi on theircontrol grids. j r

When the first negative going pulse appears, it has no action on valvesV1 and V2 but it is passed by unit 20 to the suppressor grid of valveV4. Since this valve is cut off on its control grid there is noaccompanying decrease of screen potential as in the case of the smallparticle so that no pulse is emitted to the counter.

When the second negative pulse is received there is again no action andboth units 19 and 21 are in their original stable state. It willtherefore be seen that a count is only effected when a positive goingpulse is followed by a negative going pulse produced either by thescanning or the guard beam. Thus each particle is counted twice and thetotal count must be divided by two to arrive at the number of particlesactually scanned. When a mixed signal comprising two adjacent positivepulses is received then no count is made and the switch and switchcontrol units are left in their original operative condition.

For carrying out the third mode of operation above mentioned, theapparatus shown in block schematic form in Figure 8 may be employed. Inthis arrangement the scanning and double signal producing means areindicated at 16 the signal due to the scanning beam being differentiatedas in the preceding embodiment, and only pulses of one polaritypermitted to pass to the input of a coincidence switch 22. Thedifferentiating and pulse selecting means are shown at 23 in Figure 8.The signal due to the guard beam, which may be of substantiallyrectangular form, is also supplied to the coincidence switch and thearrangement is such that when the signal due to the scanning beam isalone present the counter 9 is actuated by a pulse delivered by thecoincidence switch. When both signals are present substantiallysimultaneously the switch is prevented from being operated so that nosignal is passed to the counter.

A circuit for providing this mode of operation is shown in Figure 9. Thesignal due to the main beam, after differentiation/is applied to theanode of diode Vs, the cathode of which is connected to the control gridof triode valve V7 which acts as a pulse inverter. Positive-going pulsesof the differentiated signal are applied to the control grid of valve V7but negative going pulses are prevented from appearing on this grid.Such positive pulses provide at the anode of V7 negative going pulseswhich are applied to a bi-stable multivibrator of the same general typeshown in Figure 6. This multivibrator comprises a triode valve Va and apentode valve V9, the anode of each valve being connected to the controlgrid of the other in the usual manner. The differentiated incomingsignal is also applied to the suppressor grid of valve V9 and itscontrol grid is connected to a negative H. T. line through a resistor24, across which is shunted triode valve V10. The anode of V10 istherefore coupled to the control grid of V and has its cathode connectedto the negative H. T. line. The signal due to the guard beam is appliedto the control grid of V10. 7 I

The method of operation of this embodiment is as follows:

Initially the valve V8 is conducting and' the valve V9 is cut olf on itscontrol grid and valve V10 is non-conducting due to the connection ofits control grid to a negative bias line designated HT2 in Figure 9.

Considering first the operation with a small particle, thepositive-going pulse representing the leading edge of the particletraverses valve V6 and causes V1 to conduct so that a negative pulsefrom the anode of this valve is Valve Va is thereby cut off and themultivibrator flips to its other stable state in which V2 is conductingbeing open both on its control grid and its suppressor grid. When thenegative pulse representing the trailing edge of the particle appearsitcannot efiect the control grid of V7 due to rectifier Vs but itappears on the suppressor grid of arenas? *9 valve V9 limiting thecurrent to the anode, which causes the multivibrator to flop to itsinitial stable state. Substantially the whole of the emission current ofthe valve momentarily goes to the screen grid, the potential of whichexhibits a negative pulse which passes to the counter.

When a large particle is encountered by both beams and assuming that theguard beam encounters the particle before the scanning beam, then thecontrol grid of valve V will go positive :(assuming a positive-goingpulse) so that V10 fully conducts and reduces the potential of thecontrol ,grid of V9. When the positive pulse of the ditferentiatedsignal due to the scanning beam is applied the valve V7 and :thecorresponding negative going pulse is applied .to valve V8them-ultivibrator cannot flop since the control grid of valve V9 isbeing forceably held at a very lowpotential. Thus when thenegative-going pulse .of the scanning beam signal is applied to thesuppressor :grid of valve V9 nosignal appears on the screen grid andthus no count is registered. 7

In Figure 10 are shown (in pairs) the waveforms or the signals due tothe scanning beam and the guard beam respectively for various types ofparticles. The scanning and counting of a small particle takes placewhen the waveforms shown at (a) are applied to the coincidence switch.The waveforms at (b) are produced when a long substantially parallelsided particle, sloping in the direction of line scan, is scanned byboth beams, and at (c) when such a particle slopes in the oppositedirection. At (d) and (e) are shown the waveforms produced when aparticle having tapering sides is scanned by both beams, in the firstcase the sides are convergent in the direction of frame scan and in thesecond case divergent in the same direction.

.At (f) the guard beam alone produces a signal and in each of thesecases (b to f) no signal reached the counter since the guard beam haseither prevented the multivibrator being set or has returned it, to thecondition in which it is non-responsive to the counting signalrepresented by the trailing negative-going pulse of the differentiatedsignal due to the scanning beam.

in the preceding description specific reference has only been made toflying spot scanning means of the cathode ray tube type withspot-wobbling means for producing and distinguishing the signals due tothe two beams. It will be clear that the same result may be obtained byutilising a flying splot light beam produced by a flying spot scanner ofthe cathode ray tube type or by a light source and mirror drum or drums,or equivalent optical elements, the beam being divided by any suitableoptical element or elements. The two beams thus produced can be givendistinguishably different optical properties by being difierentlypolarised or of different colour and two photocells are then employedeach responsive to one of the beams only.

One example of such an arrangement is shown diagrammatically in Figure11 in which a light source 25 in conjunction with a condenser lens orlens system 26 illuminates a double-hole diaphragm 27. Colour filters 23and 29 which may be respectively red and blue are placed over each holeand an objective lens or lens system 30 projects the images of the twospots of coloured light into a scanning unit 31 which may be for exampleof the Well-known mirror drum type. The emergent raster-producing beamsare caused to scan the sample 32, the transmitted (or reflected) lightbeing picked up by one or other of the photocells 33, 34. Each cell maybe inherently responsive to only one of the colours and/or colourfilters 28a, 29a may be employed to ensure that one cell only sees thered beam and the other only the blue beam. If the particles are of suchcolour or colours that normal colour filters cannot be used withsuccess, interference type filters may be employed to distinguish thetwo beams.

In another alternative the raster-producing light beams,

however, initially produced, may be distinguished by being differentlypolarised that is 'to say they may be plane polarised at an angle to oneanother or differently circularly polarised, the photocells each beingmade sensitive to one of the beams only. Such means are wellknown and donot need further description.

In Figure '12 is illustrated diagrammatically another arrangement whichcomprises a flying spot scanner 35 of the cathode ray tube typeproviding a raster produced by a single electron beam in the normalmanner.

The double scanning beams for scanning the sample are produced by asplit lens system 36 one (or both) halves of the system being adjustableto enable the mutual separation of the beams to be accuratelydetermined.

As previously indicated the beams maybe distinguished by beingdifferently coloured or of difie'ren-t polarisation, colour filters orpolarising elements generally indicated at 37, 38 being disposed in thepath of the beams between the lens system 36 and the sample,corresponding filters or polarising elements 370, 38a being disposed infront of each photocell pick-up 39, 40. The two separate electricalsignals may then be employed as hereinbefore described to provide atotal count of the number of particles in the sample.

In yet another alternative form diagrammatically illustrated in Figure13 the sample suitabiy prepared is scanned directly by an electron beam.The drawing shows diagrammatically a scanning electron microscope of theknown type, having a cathode 4t, and an anode 42. provided with aperture43. The anode may therefore be considered as the source of a beam ofelectrons which is focussed on the image plane 44 by an electron lens orlens system 45'. The electron beam is deflected by a deflecting system46 to which the spot-wobbling potentials are supplied so as to produce adouble beam raster at the image plane 4-4. An electron field lens 47 atthis plane causes the electrons emergent from the plane to beconcentrated on the aperture of an electron reducing lens 48. A reducedimage of the raster at the image plane 44 is thus formed at the sampleplane 49. The sample 50 is prepared as a thin film of collodion or thelike which the electrons can penetrate and in which the film is opaqueto electrons in areas corresponding to the particles. As the sample isscanned by the electron beams the potential of a collector anode 51 willvary depending on the presence or absence of the electron beams and thisvarying potential can be employed as hereinbefore described to obtain acount of the total number of particles. With such an arrangement it ispossible to obtain a count of particles which are smaller than thosewhich can be resolved by an optical microscope and this may haveimportant uses in, for example the biological field.

It will therefore be understood that the invention may be employed forcounting particles of any kind having a random distribution on a surfaceand may be used, for example, for counting bacteria or finely comminutedmaterials of any kind provided they are suitably presented for theparticular form of scanning used.

What we claim is:

1. Particle counting apparatus comprising scanning means for providingeffectively two scanning beams simultaneously scanning adjacent paths ofa sample of the particles to be counted, pick-up means cooperating withsaid scanning means for producing an electrical signal which is ameasure of the presence and distribution of the particles, countingmeans coupled to said pick-up means and responsive 'to the derivedsignal for giving an indication of the total number of particlesscanned, and means rendering said counting means inoperative when bothbeams encounter a particle substantially simultaneously but actuatingsaid counting means when only one of said beams encounters a particlewhereby multiple counting of large particles is avoided.

2. Apparatus as set forth in claim 1 wherein said scanning means forproviding effectively two scanning beams includes means for providing amain scanning beam and means for providing a guard beam, and means formaintaining a predetermined relative positioning between said beams tocause said guard beam to scan along .a line adjacent to the line scannedby the scanning beam and in advance of it by a predetermined distance ina direction parallel to the line scanned by said main scanning beam.

3. Apparatus as set forth in claim 1 wherein said scanning meanscomprises a flying spot scanner of the cathode-ray tube type and furtherincluding spot wobbling means coupled to said scanner for repetitivelydeflecting the electron beam in a predetermined direction at arepetition frequency greater than the line frequency for producingeffectively two scanning beams.

4; Apparatus as set forth in claim 1 wherein said scanning meansprovides a flying spot beam and further including beam dividing andditferentiating means for producing two scanning beams havingdistinguishably different optical properties, said pick-up meanscomprising two photo-electric devices each responsive to only one ofsaid scanning beams.

tube type.

6. Apparatus as set forth in claim 4 wherein said scanning meansincludes a light source and a mirror drum.

7. Apparatus as set forth in claim 4 wherein said beam dividing meanscomprises a double-hole diaphragm and a lens system.

8. Apparatus as set forth in claim 4 wherein said beam dividing meanscomprises a split lens system.

9. Apparatus as set forth in claim 4 wherein said beam differentiatingmeans comprises color filters, said photoelectric devices beingcorrespondingly color sensitive.

10. Apparatus as set forth in claim 4 wherein said beam differentiatingmeans comprises polorizing means, said photo-sensitive devices beingcorrespondingly polarization sensitive.

11. Apparatus as set forth in claim 3 further including an amplifiercoupled at one end to said counting means and switching means interposedbetween the other end of said amplifier and said pick-up means,

and wherein one of said two scanning beams is a main scanning beam andthe other beam is a guard beam and wherein said spot wobbling meanscomprises an oscillator generating an alternating potential ofsubstantially square waveform, .said alternating potential being appliedto said switch means, the arrangement being such that a signal generatedby the pick-up means due to the main scanning beam is passed to oneinput of said amplifier and that due to the guard beam to another inputof said amplifier whereby when both signals are present substantiallysimultaneously one cancels the other so that there is no output signalfrom said amplifier to said counting means.

12. Apparatus as set forth in claim 2 further including means forelectrically differentiating and mixing electrical signals derived fromsaid main scanning beam and said guard beam, a switch control and countpulse generator means coupled to the input of said counting means, aswitch unit interposed between said differentiating and mixing means andsaid control and generator means, and pulse selector means coupledbetween said differentiating and mixing means and said control andgenerator means.

13. Apparatus as set forth in claim 2 further including means fordifferentiating the signal derived from said main scanning beam and acoincidence switch unit interposed between said differentiating meansand said counting means, the signal from said guard beam being appliedas a pulse of substantially rectangular form to an input of said switchunit.

14. Apparatus as set forth in claim 1 wherein said scanning meanscomprises an electron microscope having a source of an electron beamadapted to scan said sample and wherein the sample is opaque toelectrons only in areas corresponding to the particles, said pick-upmeans being a collector anode positioned on the opposite side of thesample with respect to said source of the electron beam.

Photo Cell Counts Blood Cells Faster, Popular Science, May 1949, page170.

